1. Field of the Invention
The present invention relates to a navigation system for vehicle and a route guidance method that enable a driver who is driving a vehicle on a road to safely and smoothly pass through guidance target points such as intersections.
2. Description of Related Art
A prior art navigation system for vehicle has a function of continuously providing guidance for a driver who is driving a vehicle on a road. When the vehicle has to continuously make a right-hand turn or left-hand turn two or more times while traveling within a region located at a predetermined distance from the current position of the vehicle, by using the continuously guidance function, the prior art navigation system provides guidance on a first guidance target point and simultaneous guidance on a second guidance target point, such as “Make a right-hand turn 100 m ahead and then make a left-hand turn”, before the vehicle reaches the first guidance target point. In this case, the prior art navigation system determines whether or not a distance between two successive guidance target points is equal to or less than a threshold distance and, when the distance is equal to or less than the threshold distance, provides such continuous guidance.
The above-mentioned threshold distance can be arbitrarily set. However, when the threshold distance is set to a long one, the prior art navigation system may often provide continuous guidance needlessly and therefore redundant guidance can cause the driver to have momentary lapses of attention for the first guidance target intersection at which the driver has to make a turn. In contrast, when the threshold distance is set to a short one, essentially needed continuous guidance cannot be provided and the driver may often have an inconvenience of being forced to make a sudden lane change or right or left hand turn after the vehicle has passed through the first guidance target intersection.
Thus, the setting of the threshold distance for continuous guidance is important and it is therefore desirable that the set threshold distance complies with the road on which the vehicle is traveling. In accordance with a prior art method of providing continuous guidance in consideration of the suitability of the threshold distance for the road on which the vehicle is traveling, the threshold distance for continuous guidance is properly preset according to a difference between “general road” and “highway”, each of which can be an attribute of the road between the two successive guidance target intersections (for example, refer to Japanese patent application publication (TOKKAI) No. 2000-39330).
Thus, the prior art continuous guidance method has the step of using the threshold distance for continuous guidance that is properly set according to a difference between “general road” and “highway”. However, in accordance with this prior art continuous guidance method, the threshold distance for continuous guidance is kept at constant for the road on which the vehicle is traveling, and is not set so that it suits with conditions of the road. For example, a problem with the prior art continuous guidance method is that when the vehicle is traveling on a general road, the setting of the threshold distance cannot be implemented in consideration of the configurations of various intersections on the general road. A further problem is that even when the vehicle is traveling on a highway, the setting of the threshold distance cannot be implemented in consideration of a configuration of the highway.